This invention relates to an automatic stenciling device, more commonly referred to as a silk screening jig. A variety of jigs are available designed to facilitate the alignment of a stencil screen over a workpiece to be stenciled. Such jigs are particularly useful in that a silk screening operation requires a high degree of precision, such as in silk screening printed circuits. An example of such a jig is a device manufactured by American Circuit Tool, Santa Ana, Calif., and sold under the trademark "Printmaster 5000" in which a chase framing a stencil screen is pivotally secured at a rear edge and rotatable upwardly relative to a table surface. The table is accurately adjustable to precisely position the workpiece under the screen. When the chase is in a closed position, printing fluid placed on a first end of the screen is spread by a squeegee in a print stroke and applied to the workpiece through selectively open portions of the screen. Thereafter the chase can be lifted from the workpiece a distance sufficient to enable a squeegee to sweep back the printing fluid in a flood stroke. Such a jig greatly facilitates silk screening operations, printing time and rework costs in that positioning and repeatability are assured. However, while a substantial advantage is obtained by use of such a jig, nevertheless there still remains a number of steps to be done manually, for example, raising the chase to an appropriate intermediate position, and properly applying squeegees during print and flood strokes. Particularly with the use of squeegees, manual application requires a great deal of dexterity and skill. The operator must apply appropriate pressure and use the squeegee at the proper angle, and do so time and again without substantial variation. Such high level of skill requires extensive training and experience; opertors who exhibit such skills command very high hourly rates, all contributing to high unit costs.
The present invention overcomes these drawbacks by automating the operation of such silk screening jigs. A high precision, semi-automatic screener is provided in which not only is the raising and lowering of the chase automated but the print and flood strokes are also automated, permitting accurate adjustment to desired print and flood angles, positions and pressures with reliable repeatability. As a result, ease of operation is greatly improved assuring a high quality product from personnel which can be relatively inexperienced and without the great manual skills and dexterity required of operators of manual jigs.
Certain features of the present invention have applicability to jigs regardless of how the chase is raised, whether by pivoting, as in the Printmaster 5000, or by raising from the table by any other means. Other aspects of the invention are more pertinent to a jig in which upward movement of the chase is by pivoting. In general terms, the present invention is an improvement to a jig or screener in which a chase for framing a stencil screen is movable relative to a table surface, from an open position permitting a workpiece to be placed on, or withdrawn from, the table surface, to a closed position in which the screen will contact the workpiece during a print stroke. With such jigs, a print stroke is executed when the chase is in its closed position, printing fluid on a first end of the screen being spread by a squeegee and applied to the workpiece through the screen. Thereafter, the chase is moved from the closed position and the printing fluid is swept back by a squeegee in a flood stroke. The improvement herein results in automation of operation of such a screener and comprises means for accomplishing a sequential series of steps. In particular, means are provided to move the chase to and from the open and closed positions. When the chase is in the closed position, a print squeegee is supported over the screen so that the bottom edge of the squeegee is in contact with the screen. Means are provided for retracting the print squeegee from the screen after the print stroke. At that point, the chase is raised, but upward movement of the chase is limited. Specifically, movement of the chase is limited to a distance which is a fraction of the distance from the table surface to the chase in its open position, but sufficient so that the screen is raised from the workpiece. A flood squeegee is supported over the screen and spaced therefrom so that when the screen is raised, contact is made between the bottom edge of the flood squeegee and screen whereupon the flood squeegee is driven in a flood stroke across the screen. Thereafter, the chase can be raised to the open position, whereupon the stenciled workpiece can be removed and another workpiece inserted.
While as above indicated, aspects of the invention have broad application to a jig in which the chase is raised by any mechanism, the invention is particularly applicable to a jig of the Printmaster 5000 type, in which the stencil screen is pivotally secured at a rear edge for rotatable movement upwardly relative to a table surface. In this configuration, the present invention is designed so as to prevent relative movement between the chase and the print and flood squeegees, by pivoting both the squeegee assembly and the chase around a common axis. An axial bar, defining the common axis is disposed below the rear edge of the chase extending along at least a portion of the chase. Clamps are used to rigidly secure the chase to the axial bar. The squeegees are supported by an arm which extends across the chase. The arm and squeegee assembly is rigidly secured to a bearing which is slidably and pivotally disposed on a cylindrical bearing bar extending across the rear edge of the chase and which in turn is rigidly secured to the clamp members. As a result, as the chase is raised, the squeegee and arm assembly is pivoted about the bearing bar, with both the chase and arm assembly rotating around a common axis. This arrangement also permits the squeegee-arm assembly to be swung out of the way by rotating upwardly to an obtuse angle with respect to the table when the chase is in its open or closed position.
The squeegee-arm assembly is driven by connection to a motor-driven drive chain extending along the rear edge of the chase. Specifically, the arm carries a member having at least one pin for linking with the chain. The pin-bearing member is hingedly connected to the top rear surface of the arm so that as the arm is rotated upwardly, the pin-bearing member moves in a vertically downward direction while remaining horizontal, thereby enabling the arm to be released from the chain. A mechanism is thereby provided which drives the arm in translation across the chase yet which rapidly and readily releases the arm when it is desired to swing the arm out of the way.
Both the print squeegee and flood squeegee are adjustably supported over the screen, dependent from the arm. The print squeegee is controlled by a pneumatic cylinder secured to the arm. The pneumatic cylinder has a piston shaft extending downwardly and connected to the top of the print squeegee. Means are also provided for separately adjusting the height of the front and rear edges of the print squeegee so that the bottom edge of the print squeegee can be accurately positioned on the screen. A switch is provided which is responsive to the end of a print stroke for actuating the pneumatic cylinder to retract the print squeegee.
The flood squeegee is adjustably spring-loaded on the arm and is disposed so that its lower edge is at a predetermined distance from the screen when the chase is in its closed position. This distance corresponds to a distance through which the chase is raised after a print stroke so that, following the print stroke, the screen is brought into contact with the flood squeegee simply by raising the chase. The flood squeegee is adjustable so that its front end is at a higher position than its rear end relative to the table; the angle formed by the bottom of the flood squeegee can be adjusted relative to the table to correspond to the angle defined by raising the chase said predetermined distance.
Upward movement of the chase is governed by a pair of limit members which depend from the arm and engage the table and chase at various stages. A first limit member depends from the squeegee arm directly over the forward edge of the chase. It has a follower, in the form of a roller, on its lower edge which makes contact with the chase when the chase is raised said predetermined distance. A second limit member depends from the squeegee arm at a position forward of the first limit member and extends downwardly past the edge of the chase. It too is formed with a roller as follower which engages a channel member that extends parallel to the direction of translation of the arm. The channel member can be supported by any means, such as by the jig, below the table. The channel prevents vertical movement of the squeegee arm during the print and flood strokes, but is formed with a discontinuous portion at a region which corresponds to the end of the flood stroke. At the discontinuous portion, the second limit member is released, permitting the chase to be raised to its open position. In operation, the chase is lowered to its closed position, and a print stroke is initiated, the follower of the second limit member engages the channel so that during the print stroke the squeegee-arm assembly is prevented from moving vertically while being permitted to translate across the chase. At the end of the print stroke, while the second limit member is still engaged in the channel, the chase is raised, but its upward movement is limited by the first limit member. In other words, as it moves upwardly, the upper edge of the chase engages the follower of the first limit member preventing further movement and locating the screen so that it is in contact with the bottom edge of the flood squeegee. At that point, the motor is reversed so that the flood squeegee is driven in a flood stroke across the screen. When the arm has translated sufficiently across the chase to complete the flood stroke, the follower on the first limit member meets the discontinuous portion of the channel where it is released, allowing the chase and the squeegee-arm assembly to pivot upwardly in unison.